Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, audio players, video players, personal digital assistants (PDAs), various wearable devices, mobile computing platforms such as laptop computers or tablets and/or games devices. Requirements of the mobile phone industry, for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers. Micro-electro-mechanical system (MEMS) transducers, such as MEMS microphones, are therefore finding application in many of these devices.
Microphone or pressure sensor devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive that are deposited on or within the membranes and/or a substrate or back plate. In the case of MEMS pressure sensors and microphones, the electrical output signal read-out is usually accomplished by measuring a signal related to the capacitance between the electrodes.
To provide protection the MEMS transducer will be contained within a package. The package effectively encloses the MEMS transducer and can provide environmental protection and may also provide shielding for electromagnetic interference (EMI) or the like. The package also provides at least one external connection for outputting the electrical signal to downstream circuitry. For microphones, pressure sensors and the like the package will typically have a sound port to allow transmission of sound waves to/from the transducer within the package, and the transducer may be configured so that the flexible membrane is located between first and second volumes, i.e. spaces/cavities that may be filled with air, and which are sized sufficiently so that the transducer provides the desired acoustic response. The sound port acoustically couples to a first volume on one side of the transducer membrane, which may sometimes be referred to as a front volume. The second volume, sometimes referred to as a back volume, on the other side of the one of more membranes is generally required to allow the membrane to move freely in response to incident sound or pressure waves, and this back volume may be substantially sealed. However, it will be appreciated by one skilled in the art that for MEMS microphones and the like the first and second volumes may be connected by one or more flow paths, such as small holes in the membrane, that are configured so as present a relatively high acoustic impedance at the desired acoustic frequencies but which allow for low-frequency pressure equalisation between the two volumes to account for pressure differentials due to temperature changes or the like.
The package may contain circuitry on the same or a separate semiconductor die as the membrane. The whose function of the circuitry is to measure a transducer signal related to the capacitance between the electrodes, and the circuitry may also provide one or more audio processing functions such as filtering, equalisation and the like. The integrated circuit may also provide bias to the electrodes, analog to digital conversion, analog or digital signal conditioning, an analog or digital output interface, and/or other functions.
The electrical transducer signal from the electrodes at normal sound levels is small, typically only a few millivolts. However, the supply voltage may have noise or ripple superimposed on it. For various reasons, a microphone may often be mounted at positions on the host device some way away from where the power supply voltage is generated, for example on the end of a long flex circuit to a corner of a device, or may be mounted for example under the antenna of a mobile phone where the supply voltage may be modulated by pulses of RF energy in the transmitted signal. In mobile phones, to save energy, it is common for major current-consuming blocks to be duty-cycled in operation to reduce average power consumption, for example in GSM phones with a duty-cycled RF transmitter, giving rise to time-varying changes in supply or ground connections despite reasonable attempts to mitigate these issues.
It is thus desirable to improve power supply rejection (PSR) performance of MEMS microphones such that variations and noise in a power supply voltage have little effect on any output signal from the integrated circuit. However, providing good power supply rejection has proven to be difficult to achieve in practice.